The present application generally relates to an apparatus and method for monitoring a vehicular propulsion system battery and, specifically, to monitoring the vehicular propulsion system battery to determine the presence of coolant leakage.
Hybrid and electric vehicles provide an alternative to conventional means of vehicular motive power by either supplementing (in the case of hybrids) or completely replacing (in the case of electric vehicles) the internal combustion engine (ICE). As such, at least a portion of the motive power in a hybrid or electric vehicle is provided by one or more battery packs that act as a direct current (DC) voltage source to a motor, generator or transmission that in turn can be used to provide the energy needed to rotate one or more of the vehicle's wheels. One form of battery that appears to be particularly promising for vehicular applications is known as a lithium-ion battery.
Because such battery packs form a significant part of the vehicle's propulsion system, it is important to monitor parameters associated with battery operation to ensure proper vehicular performance. Examples of such parameters include cell temperature, voltage, state of charge and so forth. Another such parameter is the coolant leakage.
Coolant leakage is an important parameter for a vehicular propulsion system battery, as it can lead to both a decrease in the efficiency of the battery thermal system as well as an increased likelihood that the system will overheat. Coolant leakage can also create a short circuit for the entire system. It can cause an isolation fault (voltage leaking from the main battery to the chassis) within the system. Finally, coolant leakage will limit the overall life of the battery.
Conventionally, a separate device is needed within a vehicular system battery to measure coolant leakage; however, one form of test known as an AC isolation resistance test can be performed both to determine whether an isolation fault has occurred and to determine coolant leakage without the need for an extra device. The conventional AC isolation resistance test is performed by injecting an excitation signal into the system to generate a readback signal. The amplitude and phase of the readback signal is determined by the difference between the excitation signal and this readback signal.
It can be difficult to ascertain the small changes between the amplitudes and phases of the original and readback signals of the conventional AC isolation resistance test, especially if the system experiences certain conditions. Even a slight difference in system conditions can lead to a large error in the isolation resistance measurement. Such a conventional method also uses a predefined range for a quantity known as Y capacitance. Y capacitors are used in high voltage systems in order to reduce interference. They are typically exposed to transients and overvoltages within a system, and are generally installed for line-to-ground or neutral-to-ground connections. Y capacitors are intended to be used where failure could lead to electric shock if proper ground connection is lost and they operate by discharging (shunting) current to the ground. The obtained value can lead to errors if the actual Y capacitance value is outside of the predefined range. The isolation resistance measurement will be inaccurate, which will lead to a false isolation detection.
Accordingly, it is challenging and difficult to accurately determine whether an isolation fault has been detected. Likewise, it is challenging and difficult to perform an accurate AC isolation resistance measurement such that the obtained Y capacitance value can be related to a correct value for important system parameters such as coolant leakage for a vehicular battery.